The invention relates generally to emitters for x-ray imaging systems and more particularly to improvements to the structures of emitters of this type.
Presently available medical x-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an x-ray tube anode, or target, which is typically a annular metal or composite structure. The space between the cathode and anode is evacuated.
X-ray tubes typically include an electron source, such as a cathode, that releases electrons at high acceleration. Some of the released electrons may impact a target anode. The collision of the electrons with the target anode produces X-rays, which may be used in a variety of medical devices such as computed tomography (CT) imaging systems, X-ray scanners, and so forth. In thermionic cathode systems, a filament is included that may be induced to release electrons through the thermionic effect, i.e. in response to being heated. However, the distance between the cathode and the anode must be kept short so as to allow for proper electron bombardment. Further, thermionic X-ray cathodes typically emit electrons throughout the entirety of the surface of the emitter. Accordingly, it is very difficult to focus all electrons into a small focal spot.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The data acquisition system then reads the signals received in the detector, and the system then translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes typically include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. An x-ray tube cathode provides an electron beam from an emitter that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce x-rays upon impact with the anode.
Typically, the cathode includes one or more cylindrically wound filaments positioned within a cup for emitting electrons as a beam to create a high-power large focal spot or a high-resolution small focal spot, as examples. Imaging applications may be designed that include selecting either a small or a large focal spot having a particular shape, depending on the application.
In these prior art x-ray tubes, the wire(s) forming the filaments are formed of drawn wire formed into coiled shape to function as the emitter. The formation of the wire in a suitable drawing process provides sufficient deformation processing to the material in order to result in a creep resistance imparted through subsequent annealing of the material. This processing, in addition to other manners of strengthening the emitter material, such as carbide-, oxide-, and/or void-strengthening the emitter material, allows the wire to have significant resistance to creep as a result of the high operating temperatures for the emitter.
Conventional cylindrically wound filaments, however, emit electrons in a complex pattern that is highly dependent on the circumferential position from which they emit toward the anode. Due to the complex electron emission pattern from a cylindrical filament or wire, focal spots resulting therefrom can have non-uniform profiles that are highly sensitive to the placement of the filament within the cup. As such, cylindrically wound filament-based cathodes are required to be manufactured having their filament positioned with very tight tolerances in order to meet the exacting focal spot requirements in an x-ray tube.
In order to generate a more uniform profile of electrons toward the anode to obtain a more uniform focal spot, cathodes having an approximately flat emitter surface have been developed, a fiat surface emitter (or a ‘flat emitter’) may be positioned within the cathode cup with the flat surface positioned orthogonal to the anode, such as that disclosed in U.S. Pat. No. 8,831,178, incorporated herein by reference in its entirety. In the '178 patent a flat emitter with a rectangular emission area is formed with a very thin material having electrodes attached thereto, which can be significantly less costly to manufacture compared to conventionally wound (cylindrical or non-cylindrical) filaments and may have a relaxed placement tolerance when compared to a conventionally wound filament.
In addition, recent developments in diagnostic x-ray tubes made it desirable to provide high emission at reduced tube voltages. For example in vascular x-ray tubes it is desirable to reduce tube voltages to 60 kV from the typical lower limit of 80 kV while ideally maintaining the power delivered to the target. For large focal spots, emission currents between 1000 mA and 1500 mA at 60 kV are desirable. For small focal spots, especially in fluoroscopic mode, emission currents up to 400 mA are desirable.
These current emitters are formed from rolled sheets of the emissive or emitter material. These sheets are formed from the same metals and/or materials utilized for the wound emitters, but are rolled into flat sheets instead of being drawn or worked into wires. These flat sheets are then cut into emitters having the desired shapes and configurations for use in x-ray tubes for more precise direction of the electrons from the emitters onto the anode/target for x-ray generation.
In rolling the material into the sheets, the amount of deformation created in the sheets is less than that created in the formation of the wires. As a result, the sheets formed of the emitter material do not have the same high temperature property benefit as found in the wound emitters. As such, under the high operating temperatures for the flat emitters, these emitters become subject to creep at lower accumulated operational times, thereby decreasing the life span of the flat emitter.
One prior art attempt to overcome this issue with flat emitters is disclosed in Falce et al, U.S. Pat. No. 7,545,089 entitled Sintered Wire Cathode, the entirety of which is expressly incorporated herein by reference for all purposes. In this reference, wires formed an emitter metal, i.e., tungsten, are wound about a bobbin and sintered in order to form the wires into a porous cathode structure including a number of desired uniform pores formed within the cathode structure as a result of the wire diameter and sintering parameters utilized.
However, the presence of the voids in the resulting porous cathode structure significantly limits the effectiveness of the cathode as a thermionic electron emitter including voids or pores in the emitter structure is detrimental to the desired emission of electrons from the emitter/cathode as well as the detrimental structural integrity impacts.
Accordingly, it is desirable to provide an emitter for an x-ray tube cathode having a flat, non-porous structure that includes yield strength, tensile strength and creep-resistance properties similar to that of wire formed or wound emitters.